Pregnancy has been termed "Nature's transplant" because the developing fetus, essentially a foreign tissue graft, is protected from rejection by its host, the mother (Editorial: "Nature's transplant" Lancet 1:345-346, 1974). Rejection of a transplanted allograft in an immunocompetent host is normally mediated by the macrophage-derived cytokine tumor necrosis factor (TNF) (Eason et al., Transplantation 59:300-305, 1995). Excessive production of TNF during pregnancy causes spontaneous abortion (Shaarawy et al., Acta Obstet. Gynecol. Scand. 76:205-211, 1997; and Mallmann et al., Arch. Gynecol. Obstet. 249:73-78, 1991). Recently, spermine, a ubiquitous biogenic amine present in large amounts in the amnion, has been shown to counter-regulate the immune response by inhibiting the production of TNF and other pro-inflammatory cytokines by human mononuclear cells (Zhang et al., J. Exp. Med. 185:1759-1768, 1997).
Fetuin is a globular 341-amino acid protein containing 20-25% carbohydrate (by weight) and 6 internal disulfide bonds. The human fetuin sequence (also known as .alpha.2-HS glycoprotein) is provided herein as SEQ ID NO. 1 and SEQ ID NO. 2. Fetuin was first identified over 50 years ago as a major protein component of bovine fetal serum but its biological function remains unclear. Bovine fetuin is a globular 341 amino acid polypeptide with six internal disulfide bonds and three N-linked and two O-linked oligosaccharide chains. Primary amino acid sequence and the position of cysteine residues are well conserved in human, bovine, sheep, rat and mouse fetuin homologs (Dziegielewska et al., J. Biol. Chem. 265:4354, 1990; Rauth et al., Eur. J. Biochem. 205:321,1992; Lee et al., Proc. Natl. Acad. Sci. USA 84:4403, 1987; and Brown et al., Eur. J. Biochem. 205:321, 1992). Fetuin levels in human plasma are regulated in the manner of a negative acute phase reactant (Lebreton et al., J. Clin. Invest. 64:1118, 1979). IL-1 was shown to suppress fetuin transcript levels in cultured hepatocytes (Akhoundi et al., J. Biol. Chem. 8:15925, 1994). Fetuin appears to be expressed in bone because transcripts have been detected in both chondrocytes and osteoblasts (Yang et al., Blood 12:7, 1991). The polypeptide .alpha.2-HS glycoprotein is a human homolog of fetuin and is secreted in high levels by adult liver into the peripheral circulation (Triffitt et al., Nature 262:226, 1976).
Human fetuin has 3 N-linked oligosaccharide chains (attached to the amine nitrogen atom of asparagine), and 2 O-linked oligosaccharide chains (attached to the oxygen atom of serine or threonine). The sugar moiety directly attached to the fetuin polypeptide is usually a N-acetylglucosamine residue. The terminal sugar residue is usually a sialic acid, in particular a N-acetylneuraminic acid (NANA) residue, which bears a net negative charge. If one removes the terminal sialic acid residue from fetuin by neuraminidase treatment, the resulting glycoprotein is an asialofetuin. Fetuin is also a carrier protein for growth factors. Fetuin is sometimes referred to as .alpha.2-HS-glycoprotein. Thus, it is considered that fetuin's biological effects on cultured cells are related to its carrier function for molecules with growth-promoting properties.
The synthesis of human .alpha.2-HS-glycoprotein is down-regulated by cytokines (hIL-1.beta., hIL-6) (Lebreton et al., J. Clin. Invest. 64:1118-1129, 1979). Human fetuin levels are decreased (25-50%) in trauma patients (van Oss et al., J. Trauma 15:451, 1975). Therefore, there is a need in the art to find a utility for fetuin and to understand fetuin's physiological role and the importance of its many negatively charged (at physiologic pH) sialic acid residues.